From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7BD7AC636D6 for ; Thu, 23 Feb 2023 15:34:45 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pVDcD-0004Ee-Hc; Thu, 23 Feb 2023 10:34:37 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pVDc4-0004BB-Jn for qemu-devel@nongnu.org; Thu, 23 Feb 2023 10:34:28 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pVDc2-0005HU-QG for qemu-devel@nongnu.org; Thu, 23 Feb 2023 10:34:28 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1677166464; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ur1gfoLT0cUMevlkAHvJopiA7UrE2IWfgPbAJ0H8WDo=; b=h/bZEbOvBJBvjNUgfecfMTUmcgXjVXOJFDPXYv9BB7pXCgdJTft1uvKfx/anXlh4S5ySK2 WyXU7bKj+eXT8uhGyH9pIxrmA4m9vXQzyTWcwoZWVteIHbJkm0sC4EVoFsPk2olYkZTU3r bzMdCHEApOSIxQ8Iz7i4MWAkhBEWQq0= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-459-4fdaQIjHMiq0dBOF1z358w-1; Thu, 23 Feb 2023 10:34:22 -0500 X-MC-Unique: 4fdaQIjHMiq0dBOF1z358w-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id D7D63101A52E; Thu, 23 Feb 2023 15:34:21 +0000 (UTC) Received: from localhost (unknown [10.39.192.157]) by smtp.corp.redhat.com (Postfix) with ESMTP id 107091121315; Thu, 23 Feb 2023 15:34:20 +0000 (UTC) Date: Thu, 23 Feb 2023 10:03:32 -0500 From: Stefan Hajnoczi To: Stefan Hajnoczi Cc: Kevin Wolf , qemu-devel@nongnu.org, qemu-block@nongnu.org, "Michael S. Tsirkin" , Paolo Bonzini , David Hildenbrand , Peter Xu , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , Fam Zheng , Qing Wang Subject: Re: [PATCH v2 3/3] virtio-scsi: reset SCSI devices from main loop thread Message-ID: References: <20230210143238.524357-1-stefanha@redhat.com> <20230210143238.524357-4-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="zqQ5fO04DIRfWZOW" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 Received-SPF: pass client-ip=170.10.129.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --zqQ5fO04DIRfWZOW Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Feb 21, 2023 at 11:40:26AM -0500, Stefan Hajnoczi wrote: > On Fri, Feb 17, 2023 at 11:22:02AM +0100, Kevin Wolf wrote: > > Am 10.02.2023 um 15:32 hat Stefan Hajnoczi geschrieben: > > > When an IOThread is configured, the ctrl virtqueue is processed in the > > > IOThread. TMFs that reset SCSI devices are currently called directly > > > from the IOThread and trigger an assertion failure in blk_drain(): > > >=20 > > > ../block/block-backend.c:1780: void blk_drain(BlockBackend *): Asse= rtion `qemu_in_main_thread()' failed. > > >=20 > > > The blk_drain() function is not designed to be called from an IOThread > > > because it needs the Big QEMU Lock (BQL). > > >=20 > > > This patch defers TMFs that reset SCSI devices to a Bottom Half (BH) > > > that runs in the main loop thread under the BQL. This way it's safe to > > > call blk_drain() and the assertion failure is avoided. > >=20 > > It's not entirely obvious what is the call path that leads to > > blk_drain(). Do we somehow call into virtio_scsi_dataplane_stop()? >=20 > I'll make it clearer that resetting the SCSIDevice calls blk_drain(): >=20 > void scsi_device_purge_requests(SCSIDevice *sdev, SCSISense sense) > { > SCSIRequest *req; >=20 > aio_context_acquire(blk_get_aio_context(sdev->conf.blk)); > while (!QTAILQ_EMPTY(&sdev->requests)) { > req =3D QTAILQ_FIRST(&sdev->requests); > scsi_req_cancel_async(req, NULL); > } > blk_drain(sdev->conf.blk); > ^^^^^^^^^^^^^^^^^^^^^^^^^^ >=20 > The IOThread code path is virtio_scsi_handle_ctrl_req -> > virtio_scsi_do_tmf -> device_code_reset -> scsi_disk_reset -> > scsi_device_purge_requests. >=20 > > > Introduce s->tmf_bh_list for tracking TMF requests that have been > > > deferred to the BH. When the BH runs it will grab the entire list and > > > process all requests. Care must be taken to clear the list when the > > > virtio-scsi device is reset or unrealized. Otherwise deferred TMF > > > requests could execute later and lead to use-after-free or other > > > undefined behavior. > >=20 > > Why don't we already need the same for other asynchronously processed > > requests? Certainly having a read request write to guest memory after > > the device has been reset or unplugged isn't what we want either. >=20 > I/O is already taken care of because bdrv_drain_all() is called in > do_vm_stop(). TMFs are not purely a device emulation concept and not a > block layer concept covered by bdrv_drain_all(), so they need to be > handled explicitly. >=20 > >=20 > > I see that we assert(!s->dataplane_started) in virtio_scsi_reset(), > > which may be part of the reason. If we're not processing any requests, >=20 > Yes, dataplane is stopped when the virtio-scsi device is reset. >=20 > > then we're safe. virtio_scsi_dataplane_stop() calls blk_drain_all() > > (which is really a too big hammer) in order to make sure that in-flight > > requests are completed before dataplane_started becomes false. > >=20 > > I was wondering if we couldn't just blk_inc_in_flight() while a TMF > > request is in flight and then use the same draining logic to be covered. > > You could use oneshot BHs then and do away with the list because you > > don't need to cancel anything any more, you just wait until the BHs have > > completed. > >=20 > > The practical problem may be that we don't have a blk here (which is > > probably also why blk_drain_all() is used). We could have our own > > AIO_WAIT_WHILE() instead. I feel waiting instead of cancelling BHs would > > simplify the code. > >=20 > > In fact, I think technically, you may not need any of that because > > blk_drain_all() also executes all BHs in the main loop before it > > returns, but that might be a bit subtle... >=20 > There is a bit of a layering violation if we use the fake block layer > requests to track virtio-scsi HBA TMF emulation. >=20 > The starting point for doing this would probably be a > SCSIDeviceClass->schedule_reset() function that allows scsi-disk.c to > implement the one-shot BH + blk_inc_in_flight() approach. scsi-disk.c > has the BlockBackend. >=20 > The fact that we're having to think hard about how to make this work > makes me wonder whether it's a good idea? >=20 > > > The s->resetting counter that's used by TMFs that reset SCSI devices = is > > > accessed from multiple threads. This patch makes that explicit by usi= ng > > > atomic accessor functions. With this patch applied the counter is only > > > modified by the main loop thread under the BQL but can be read by any > > > thread. > > >=20 > > > Reported-by: Qing Wang > > > Cc: Paolo Bonzini > > > Signed-off-by: Stefan Hajnoczi > >=20 > > Kevin > >=20 --zqQ5fO04DIRfWZOW Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmP3gEQACgkQnKSrs4Gr c8jA8Af/R8VbaCjEDqZQFgqGpuQMCEk45n5l3dxHh+bcxEHUukV2yuVJJlw7kTIR q5XHQ6JS1mOkFNrpWfxxD+US7GMPWSao91NV3RZKeIXx6ms88LTV3c8sNOoDVGgA d1FglW1LR9mNczyfXc6E6twHFnEbzL8KgF3b7UTu5zm7oAZUGhHCsJClipvCaY6o FJEvrpa12GR7UPA/0N0ULUpp2U2FItnCatpd1vcK8oXYmEcLj4C2Yn6qr9M/A48u 2dJOvSt++YqYbYLRF7W0AdNfb3WF1fpCxv6zJ8TQcPukNNz158HKy8PJTl/8kXLx 6qaMIWuLdDndpHGMwX2CN9Yv+49F5A== =th6W -----END PGP SIGNATURE----- --zqQ5fO04DIRfWZOW--